This invention relates to a multilayer structure, and more particularly to a multilayer thermoformable packaging material including a barrier layer and at least one layer which utilizes scrap produced from the multilayer structure.
Processes for molding or otherwise shaping plastic sheets or film into containers, packaging, and the like utilizing thermoforming techniques have been available for many years. A typical thermoforming process heats a thermoplastic sheet to its softening point and then shapes the sheet at a forming station utilizing various molds and/or vacuum or air pressure assists. Other thermoforming techniques include solid phase pressure forming (SPPF) and the so-called scrapless forming process (SFP). For purposes of this disclosure, references to thermoforming techniques or thermoformable structures will include SPPF and SFP.
Olefin polymers, such as polyethylene and polypropylene, offer desirable physical properties in container and packaging design such as high heat distortion temperatures, toughness, and resistance to environmental stress cracking. However, in part due to their relatively sharp melting points and low melt strength, polyolefin homopolymers or coextruded homopolymer structures cannot be practically thermoformed on conventional standard equipment.
One approach to this problem is illustrated by Grancio et al, U.S. Pat. Nos. 4,386,187 and 4,386,188. Grancio et al teach a thermoformable polymer blend of a polyolefin, such as polyethylene, with polystyrene and a compatibilizing styrene-butadiene-styrene block copolymer. The polystyrene component of the blend improves the processability of the composition so that it can be readily thermoformed.
Other container and packaging applications require a plastic film which has low moisture vapor and gas transmission rates, i.e., so-called barrier films. Such barrier films are particularly desirable in the packaging of foods such as meats and cheeses. However, plastics which provide good barrier properties in many instances do not possess properties necessary for conventional thermoforming techniques. Thus, the prior art has utilized multilayer plastic film structures incorporating a barrier layer in an attempt to overcome this problem.
For example, Peterson, U.S. Pat. No. 3,524,795, teaches a thermoformable multilayer packaging material employing polyolefin outer layers with an inner barrier layer of Saran; the layers are adhered together with a bonding agent such as a copolymer of ethylene and vinyl acetate. Other prior art teachings of thermoformable multilayer containers and films include French Pat. No. 1,401,433; Monia, U.S. Pat. No. 3,793,135; Valyi, U.S. Pat. Nos. 3,955,697 and 4,048,361; Wiggins, U.S. Pat. No. 4,057,667; Yoshikawa et al, U.S. Pat. No. 4,161,562; and Catte, U.S. Pat. No. 4,234,663. Many of these references also teach the use of bonding agents to adhere incompatible layers together.
However, the use of thermoformable multilayer materials often results in considerable quantities of scrap being formed from the processing operations. Such scrap either cannot be recycled or is at best difficult to recycle because the various layers normally have incompatible plastic resin compositions. One approach to this scrap problem is exemplified by Haefner, U.S. Pat. No. 3,940,001 and Schrenk, U.S. Pat. No. 3,977,153. There it is taught to separate the loosely adhered individual layers prior to recycling.
However, such an approach is feasible only in those instances where the multilayer structure is useable in applications where the individual layers need not be securely adhered together. In those instances where the multilayer structure has been formed as true laminate, the layer cannot be separated for recycle. The Catte patent, listed above, teaches that scrap from a multilayer film can be incorporated as a component of a layer positioned intermediate of layers of polyolefin and a polystyrene, respectively. However, Catte teaches the necessity of combining a bonding agent with the scrap to form the intermediate layer. Moreover, none of the layers of Catte acts as a barrier layer.
Accordingly, the need still exists in the art for a multilayer thermoformable material which utilizes scrap produced from the multilayer structure. In addition there is a need for such a material which utilizes scrap and which incorporates a barrier layer. Further, there is a need for such a material utilizing scrap in which the individual layers are compatible and may be adhered directly to one another without the need for an intermediate bonding agent.